North Korea's Musudan intermediate range ballistic missile, also known as Hwasong-10, entered service in the Korean People's Army Strategic Force in mid 2016. The missile closely resembles the country's Pukkuksong-1 submarine launched ballistic missile and is widely speculated to be an advanced land based adaptation of the formidable platform. The Musudan represented one of the first of a new generation of North Korean missiles, alongside the Hwasong-12, Hwasong-14, Hwasong-15 and Pukkuksong-2. These platforms all notably built on the basis technologies developed for less sophisticated predecessors which marked the early stages of the country's missile program - platforms such as the SCUD-C, Rodong and Taepodong-2.
Within two years of the Musudan's induction, the Islamic Republic of Iran unveiled its own intermediate range missile - the Khorramshahr. The missile was named after the port city of Khorramshahr, capital of a county of the same time, near the Iraqi border. The city was the site of a key battle of the Iran-Iraq War, under which the Iranian Revolutionary Guard evicted a large Iraqi force and repulsed a later Iraqi counterattack. The Iraqi army was demoralized by this defeat, with 19,000 of its army captured as prisoners of war. May 24th, the day of their defeat, is still known by Iraqis today as 'bad luck day.' The Iranian victory at Khorramshahr played a pivotal part in the war and was key to turning the tide against Iraqi forces.
Iran has often justified its pursuit of ballistic missiles by referring to its inability to defend itself in the face of the Iraqi, which made extensive use of ballistic missiles and weapons of mass destruction against Iranian civilian targets. As such it was only fitting that one of its most potent missiles was named after a key battle of the Iran-Iraq War. The Khorramshahr however is not an entirely indigenous design, but rather is closely based on North Korean Musudan missiles sold to Iran as part of their extensive military cooperation. Indeed, the Khorramshahr is hardly the first Iranian platform to be based closely on a North Korean design. Perhaps the most prolific case of this was is the Iranian Shahab-3 short range ballistic missile, one of the country's most numerous platforms, which was developed based on the Korean Rodong-1. The Islamic Republic not only purchased Rodong-1 missiles from their North Korean partners, but also gained extensive Korean assistance in developing the infrastructure domestically and acquiring the technology needed to produce such systems in Iran itself.
Wheather the Khorramshahr missiles currently in service were produced domestically by Iran with North Korean assistance, or weather they are Musudan missiles which have been modified to suit Iranian requirements, remains to be seen. It is possible that the latter is the case for missiles so far observed, and Iran is displaying modified North Korean made platforms only until it acquires the infrastructure and technology needed to produce its own.
One notable difference between the Khorramshahr and Musudan is that while the North Korean platform retains a 3,500-4000km range, the range of the Khorramshahr is according to the Iranian military only 2000km. There are two likely explanations for this difference. The first is that Iran is understating the range of its missile to avoid concerning European nations which would be in range of Iranian missile strikes should it possess capabilities similar to those of the Musudan. With Iran attempting to maintain positive ties with Europe, particularly following the JCPOA nuclear deal reached in 2015, the Rouhani seeks to prevent European powers from taking a stronger stance against the country. European relations serving as an incentive for Iran to understate its missile range has been speculated by numerous Western analysts.
The second hypothesis for the shorter stated range of the Khorramshahr missile relative to that of the Musudan is one of applicability. North Korea has since very the beginning of its missile program been renowned for its ability to modify missiles to maximize their range at the expense of payload. This is an invaluable asset for the country to be able to strike its targets, which other than U.S. facilities in South Korea stretch from Okinawa to Guam to the U.S. East Coast. The Musudan is capable of striking targets across Japan as well as U.S. facilities on Guam 3,400km away. For Iranian use however, the furthest potential target is Israel which lies well under 2,000 kilometers away - with other targets including U.S. military facilities in the Persian Gulf and Afghanistan and potentially Saudi Arabian and UAE military facilities as well, all of which are significantly closer to Iranian territory than Israel. As a result is that Iran does not need a missile which can reach 4,000 km, as primary targets between 2000km and 4000km away are relatively few. The Khorramshahr may well therefore have been modified with the reverse of the modification usually done by the North Koreans, to compromise range and increase payload - or even decrease size. This remains highly likely, and the Khorramshahr is as a result a essentially a modified variant of the North Korean Musudan tailored to suit Iran's defence needs. It represents a recent manifestation of the two countries' longstanding relationship in the military field from which both parties continue to benefit substantially.